1. PCB DESIGN - A QUICK WALK THROUGH
SREEKANTH N NAMPOOTHIRI

2. Agenda Introduction PCB Over View E-CAD Tool Sets
PCB Design Life Cycle
PCB Manufacturing
Common Design Tips
Latest technologies in PCB
PCB Dictionary

3. Introduction ‘PCB’ stands for Printed Circuit Board Design
PCB, is used to mechanically support and electrically inter connect components using conductive pathways called as traces, etched from copper sheets laminated onto a non-conductive substrate
PCBs were designed and laid out by hand using adhesive tapes & pads on clear drafting film
ECAD system is developed historically to engineer the electronic circuit boards more efficiently
The PCB design never much affected circuit performance in the past where the speed of operations were too slow. But Industry demands real engineering expertise on PCB nowadays where the rise time of silicon fell into Pico second ranges.
Modern day examples of the high speed signals include DDR - DDR2 Bus, USB, SATA, PCI Express, PCI, Gigabit Ethernet etc. High speed design techniques must be applied to the printed circuit boards containing these and other high speed signals to ensure proper and reliable operation
Engineers are assigned the task of design mixed analog and fast rise-time digital boards, and have all signals arrive at their destination cleanly 3

4. PCB OverView
PCB is a stack of Different layers-Cu foils, Cores, Prepregs,Solder Mask& Silk screen
Laminate Cores (with both side copper sheets) & Copper foils are used to construct Electrical Layers ) (Eg :- FR-4)
Pre-preg material is used as isolation layer between two electrical layers.
General standard on PCB is IPC-2221A
Cross sectional View of a Multi Layer PCB 4

5. E-CAD Tool Sets
There are many EDA vendors in the industry who have developed Electronics-CAD tool chains.
The major players are Cadence, Mentor Graphics,Zuken,Altium & the tools are :
Allegro from Cadence
Board Station,Expedition,PADS etc from Mentor Graphics
Visula,Cr5000,CadStar etc from Zuken
Pcad/Altium Designer from Altium 5

6. PCB Design Life Cycle ECAD Library Development Schematic Entry
Net-list porting
Critical net analysis & Rule settings
Component Placement & BGA fanout
Routing
Design Rules, DFM,DFA verification
Post processing
Gerber , Drill files & Supportive files Generation 6

7. PCB Design Life Cycle ECAD Library Development
ECAD Library contains of electronic parts which has a schematic Symbol,a PCB footprint and an association of both. It shall be developed as per IPC standards & recommendations Many companies nowadays maintain a standardised master library management system.
Schematic Entry
Schematic Entry is the process of converting the logical electronic design to a pictorial representation using symbols, nets etc.
Net-list porting
It is the process of creating a net list file and port the same to a PCB design environment where all the logical symbols are mapped to PCB footprints.
Critical net analysis & Rule settings
High speed interfaces/nets are identified and different placement/routing rules are set up.
Component Placement & BGA fanout
Floor planning & component placements plays very critical role in PCB engineering Logical grouping of components are done.DFA/DFT principles are applied in component placement.BGAs are fanned out prior to the routing face. 7

8. PCB Design Life Cycle Routing
Routing is the process of establishing the physical path for conducting electricity in a printed circuit board. Different electrical rules need to be applied while routing high speed tracks, differential pair routing, GND-POWER splitting etc.The main routing topologies are Single ended, Differential pair, Daisy Chain,Star,T etc
DRC
Design Rule Check is the process of verifying all the rules such as min track width,space constraints.
Post processing
After verifying the 100% routing completion & electrical DRC clearance, the design data needs to be prepared with silk screen alignment, solder masking,Fab notes updation etc etc.
Gerber & drill files Generation
RS274-EX format is the accepted Gerberfile format and all the layer’s gerber files are generated seaparately.The drill files are generated in excellon format
All the files will be provided to the PCB vendor for starting the manufacturing process.
A set of files like Drill & Assmebly drawings,Centroid data,Solder paste data etc also generated from the design file 8

9. PCB Manufacturing What Happens to Gerber files ?
Load the Gerber & drill files in CAM station
Perform CAM analysis & final layer stack up
Generate uniform Solder Mask Data
Perform Panalization
Add common fiducials & create test coupons, if required.
Core Laminate is selected in required thickness & panel Size
Photo Resist material is applied to both side of the core laminate
Develop the –ve image of the inner layers in core laminate by exposing with UV light
Wash off the unhardened photo resist material
Etch off the unwanted copper
Remove the photo resist materials on the track & pads 9

10. PCB Manufacturing Continues…
Automatic Quality Inspection (AQI) is done
All the inner layer etching is completed same way
Outer layer copper foil is added to the inner core stacks with pre-preg isolation
All Electrical layers are stacked & pressed in pressing machine
Panel is gone for CNC drilling,PTH plating etc
Outer Layers are developed & Solder Mask is applied.
The panel is taken for Surface finish process such as HASL,ENIG etc
Bare Board Test (BBT) is performed on the panel with a Flying probe tester
De-panellize & ship the boards after final QA . 10

11. Latest Technologies in PCB Design
Use of uBGAs,SOCs
Mixed Signal technologies- Analog,Digital,RF
Controlled Impedence PCBs
Use of micro,blind/Buried Vias
Back drilling
Embedded passives
Flexible PCBs
Optical PCB tracks 11

12. Common Design Tips Tips for Improving Signal Integrity
For all high speed signals use controlled impedance traces
Place series termination resistor as close to the driver IC as possible.
Match the timing relation between clock and data signals using length and time-delay calculations. Account for the fact that signals travel faster in microstrip than in stripline
Keep the routing of the differential signals symmetrical. Keep the number and placement of the vias symmetrical.
If multiple powers share a power plane, and if the power plane is used as a reference plane for return signal of high speed traces use stitching capacitors to “stitch” the boundaries of the power plane islands.
Tips for Avoiding Cross Talk
Give preference to stripline over microstrip when routing critical signals sensitive to cross talk
Reduce the separation between the signal and ground layers as far as possible while still achieving the required impedance
Space out the critical high speed traces as much as possible. A minimum spacing of twice the signal width is a good number. . 12

13. Common Design Tips Continues…
If guard trace is used, use vias to connect the guard trace to ground
Make sure traces in adjacent layers run perpendicular to each other
Tips for Power Supply Noise Reduction
Keep the separation between power and ground plane as small as possible
Use wider traces & large vias to connect decoupling capacitors to power and ground planes
Place decoupling capacitors as close to the IC power pins as possible
Use large number of decoupling capacitor to reduce total effective series inductance
Tips for EMI Reduction
Keep the power and ground signal layers as close as possible
Keep all traces that carry high frequency and fast edge rate signals in stripline in place of microstrip & well inside the edge of the board
Make provision for a ferrite bead at the entry path of power supply to reduce the conducted emission . 13

14. PCB Dictionary PCB – Printed Circuit Board Design
PTH-A hole in a circuit board that has been plated with metal (usually copper) on its walls to provide electrical connections between conductive patterns
Via – A PTH that electrically connects the signal between two/more layers
Annular Ring – The conductive material surrounding a hole
Etch -Chemical removal of metal (copper) to achieve a desired circuit pattern
Ground Plane-A conductor layer, or portion of a conductor layer, used as a common reference point for circuit returns, shielding, or heat sinking
Mil - One-thousandth of an inch (0.001").
DFM-Design for Manufacturability
DFT-Design for Testability
DFA-Design for Assembly 14

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